The present invention relates to an apparatus and method for the concentration of ore by flotation. In particular, it relates to the concentration of an ore by flotation wherein a portion of the ore is coated with a beneficiating amount of a flotation reagent and the reagent-coated particles form a concentrate floating at a higher level in the float medium than the non-coated particles due to aeration of the conditioned ore slurry. More particularly, it relates to an apparatus and method for the concentration of coarse phosphate rock by agglomerative flotation.
Flotation has been used since the early 1900's as an important part of the ore concentration process. For example, more than two-thirds of the phosphate rock produced in Florida is upgraded by the flotation process. Since no crushing or milling steps are involved in the material preparation, the flotation plant feed is coarser than in most other mineral flotation systems. During hydraulic processing of the rock, particles in the size range of -14 to +150 mesh constitute the flotation feed. This size fraction is further classified by hydraulic sizers, stationary screens and vibrating screens into a coarse -14 to +35 mesh and a fine -35 to +150 mesh flotation feed. The flotation techniques for concentrating the fine feed as practiced by the Florida phosphate producers have generally been standardized over the years, however, methods for upgrading the coarse feed differ within the industry.
In the past, treatment of the coarse feed fraction by conventional flotation machines has been found to be inefficient and uneconomical because of low recoveries. Efforts were made to reduce the coarse size fraction by hammer mills, but this was found to be uneconomical and was discontinued.
The lower recoveries in conventional flotation cells have been attributed to the inability of air bubbles to carry the relatively larger particles up to the froth column. Generally, recovery of particles coarser than 28 mesh is poor when employing conventional flotation techniques for upgrading the coarse feed.
The three flotation separation processes, froth flotation, agglomeration flotation and skin flotation are based upon the same fundamental principle of selective adhesion. They differ from each other in the physical means that are employed to achieve the desired separation. In froth flotation, or conventional flotation, small mineral particles attach to gas bubbles in such a way that the specific gravity of the "loaded" gas bubble is less than that of the pulp. This results in loaded bubbles rising through the pulp and forming a mineralized froth. The mineralized froth is then mechanically separated from the pulp.
In agglomeration flotation, mineral particles are loosely bonded with relatively smaller gas bubbles to form agglomerates. These agglomerates are denser than water but less dense and larger than particles wetted with water. Separation of the agglomerated and non-agglomerated particles is achieved by flowing film, gravity concentration. The particles which can be separated by agglomeration are substantially coarser than those separated by froth flotation. When brought into contact with a free water surface, agglomerates are replaced by skin floating individual particles.
In skin flotation, surface tension forces result in flotation of the non-wetted particles. The particles which are wetted with water sink and therefore can be separated from the particles floating at the gas-liquid interface. Coarser particles can be concentrated using skin flotation than can be efficiently handled by froth flotation.
The basic property of selective adhesion, in all three separation processes, is based on the natural or induced flotability of the mineral particles. The degree of flotability of the mineral particles can be modified by selective absorption of surface active agents. Surface tension forces are also important in all three separation processes.
For concentration of coarse phosphate, most techniques other than froth flotation employ a combination of agglomeration and skin flotation processes. Even though the agglomerates formed in the pulp are denser than water, they rise to the gas-liquid interface because of the hydrodynamic forces generated in the pulp. Once the agglomerates reach the gas-liquid interface, they skin float and become separated from the non-agglomerating particles which remain suspended in the pulp.
Lang launders, belt flotation and spiral flotation are currently employed for concentrating coarse phosphate feed. The Lang launder method is disclosed in U.S. Pat. No. 3,326,373. The belt flotation method was developed in the late 1930's. A belt flotation method and apparatus is disclosed in U.S. Pat. No. 2,047,773 to Green and Wilbur. Spiral flotation was developed and has been in use since the late 1940's. Table flotation has been utilized in the separation of coarse phosphate rock since about 1934 but its use has diminished. A process for table flotation is described in U.S. Pat. No. 1,968,008 to Chapman and Littleford. In all these methods, separation is achieved by skin flotation of the phosphate agglomerates. Separation of phosphate agglomerates from gangue minerals is achieved by sizing in underwater agglomerate screening, which was one of the earliest methods devised for recovery of phosphate from -20 mesh material. Underwater agglomerate screening utilizes oil agglomeration of coarse phosphate particles in the absence of aeration. Such processes are taught in U.S. Pat. No. 2,017,468 to McCoy, Wright and Hall and U.S. Pat. No. 2,113,727 to Hall and Hodges.
Froth flotation is also practiced on coarse phosphate feed and is similar to rougher flotation of fine phosphate feed. Low recoveries of coarse phosphate rock in froth flotation has led to the use of larger flotation cells to increase efficiency. Such larger flotation cells include 50 cubic feet cells made by Denver Equipment Co., 100 cubic feet cells made by Galigher Co. and 300 cubic feet cells made by Wemco division of Envirotech Corp.
The methods currently in use for separation of coarse phosphate rock have their disadvantages and advantages. For example, during froth flotation in the large flotation cells, of the +35 mesh fraction feed to the cells, less of the total coarse feed is recovered. Major limitations of table flotation are excessive maintenance of the equipment and the need for precise control of the feed size. In underwater agglomerate screening, large amounts of reagents are required to achieve satisfactory separation and strict control of feed size is required. Of the three techniques presently being employed for coarse feed concentration, belt flotation has a relatively lower throughput capacity for the floor space required. In spiral flotation feed size control is essential to achieve desired separation. Rougher concentrate from Lang launders, belt flotation and froth flotation circuits has to go through a cleaner circuit to produce a final concentrate. The concentrate produced from spiral flotation and table flotation generally does not require any further cleaning. These techniques are further described in the paper entitled "Agglomeration-Skin Flotation of Coarse Phosphate Rock," by Brij M. Moudgil and Dave H. Barnett, presented at the annual meeting of AIME in Atlanta, Georgia, Mar. 6-10, 1977.